Why do you sometimes need to define a destructor as a virtual function?

Why do you sometimes need to define a destructor as a virtual function?

The function of the destructor is to do the necessary cleanup work before the object is revoked, when the object of the derived class is revoked from memory, the destructor of the derived class is called, and then the destructor of the base class is called, but a derived class object is established with the new operator, and it is assigned to the base class pointer. Then when the delete operator is used to undo the storage space pointed to by the pointer, the system executes only the destructor of the base class, not the destructor of the derived class.

Why do you sometimes need to define a destructor as a virtual function?